1. Field of the Invention
The present invention generally relates to semiconductor electronic switches, and more particularly to high power, high temperature gate-assisted turn-off thyristor devices, that use a multi-layered thick drift region to increase the voltage blocked.
The invention described herein may be manufactured, used and/or licensed by or for the United States Government.
2. Description of the Related Art
Thyristors are bistable power semiconductor devices that can be switched from an off-stale to an on-state, or vice versa. Thyristors, along with other power semiconductor devices such as high-power bipolar junction transistors and power metal oxide semiconductor field effect transistors control or pass large amounts of current and block high voltages. Unlike conventional thyristors, a gate turn-off (GTO) thyristor is turned off by a reverse gate pulse. Generally, a trigger input implements conduction in GTO thyristors. Thereafter, the GTO thyristors behave as diodes.
A thyristor is a very rugged device in terms of transient currents, di/dt, and dv/di capability. The forward voltage (Vf) drop in conventional silicon thyristors is approximately 1.5 V to 2 V, and for some higher power devices, it is approximately 3 V. Therefore, a thyristor can control or pass large amounts of current and effectively block high voltages (i.e., a voltage switch). Although Vf determines the on-state power loss of the device at any given current, the switching power loss becomes a dominating factor affecting the device junction temperature at high operating frequencies. Because of this, the maximum switching frequencies attainable using conventional thyristors are limited, as compared with many other types of power devices.
Two important parameters for a thyristor are the built-in potential (which is a characteristic of any semiconductor material""s bandgap) and the specific on-resistance (which is the resistance of the device in the linear region when the device is turned on). Preferably, the specific on-resistance for a thyristor should be as small as possible so as to maximize the current per unit area for a given voltage applied to the thyristor. In particular, the lower the specific on-resistance, the lower the Vf drop is for a given current rating. Moreover, the minimum Vf for a given semiconductor material is its built-in potential (voltage).
Conventional thyristors may be made of silicon, for example, such as a silicon-controlled rectifier. However, thyristors made of silicon have certain performance limitations inherent in the silicon material itself, such as the thickness of the drift region. The largest contributory factor to specific on-resistance is the resistance of the thick low-doped drift region of the thyristor. Typically, as the rated voltage of a thyristor increases, the thickness of the drift region increases and the doping of the drift region decreases. Thus, the resistance of the drift region increases dramatically. Therefore, the thickness of the drift region should be minimized and the level of doping should be maximized, for any given rated voltage so as to minimize the specific on-resistance for the device.
Several thyristor structures have been developed in an attempt to solve the on-resistance problems described above. These conventional devices include variations of the silicon material to try to lower the on-resistance. However, the conventional devices are limited by the inherent characteristics of the silicon semiconductor material itself. The electrostatic breakdown field is lower in silicon than it is in silicon carbide. This, in turn, requires that these portions be physically thicker, which makes for a generally disadvantageous specific resistance.
Silicon carbide offers a number of advantageously unique physical and electronic properties, which makes it particularly useful for thyristors. This includes its high melting point, high thermal conductivity, radiation hardness (particularly to neutron radiation), wide bandgap, high breakdown electric field, and high saturated electron drift velocity. With the high breakdown field, thinner devices can be developed that block a given amount of voltage compared to silicon devices. This provides faster switching devices because a smaller volume of charge carrier must be removed during turn-off. Therefore, the power handling capability of silicon carbide GTO thyristors is much better than that of silicon GTO thyristors. Additionally, silicon carbide is physically rugged and chemically inert.
Because of the superior physical and chemical properties of silicon carbide, several applications for silicon carbide GTO thyristors exist including high voltage DC systems, traction circuits, motor control, power factor control, and other power conditioning circuits. Moreover, these systems may be found in electric or hybrid electric vehicles, including tanks and helicopters. However, one major limitation to the use of conventional silicon carbide GTO thyristors in these systems is the low turn-off gain. Turn-off gain is the ratio of the cathode current being switched off by the GTO thyristor divided by the maximum gate current required for the switching process. If the turn-off gain is low, then the switching losses will be high, thereby reducing the amount of power that a single silicon carbide GTO thyristor can handle. Also, the circuit providing the gate drive current to turn off the GTO thyristor requires extremely robust devices, which increases the cost and complexity of the overall system.
Conventional approaches have attempted to provide a device capable of blocking substantially high voltages. In particular, silicon carbide GTO thyristors have been used to try to achieve these results. Unfortunately, the conventional devices have not achieved the level of satisfaction sought. In order to prevent the breakdown of the voltage blocking capability, techniques are required to reduce the maximum electric field that occurs in the high voltage device at a given voltage. Conventional approaches for silicon GTO thyristors include field rings, field plates, beveling, and ion implanted edge terminations, which have been tested in silicon carbide GTO thyristors. However, the performance and yield in large batches of these conventional devices remain unacceptable. While silicon carbide GTO thyristors that block up to 3.3 kV have been implemented, these devices use a 50 micron thick drift region to support this high voltage, wherein such a thickness would indicate that these devices should actually block over 6.5 kV. Thus, the conventional devices are underachieving. Also, this maximum value of 3.3 kV has shown to be inconsistently achievable. Furthermore, the thick material is difficult to grow, leading to rough surfaces and poor electrical characteristics such as low free carrier lifetimes and mobility (electron and hole mobilities).
Conventional methods and devices for increasing the amount of voltage that can be blocked in a bipolar device use a shallow etch and a field stop on a pin structure. Unfortunately, this brings the maximum value of the electrostatic field near the surface of mesa isolated devices. This causes disadvantageous effects because physical corners, such as those between the mesa and the flat silicon carbide regions outside of the mesa, create mesa isolation, and further concentrate the electrostatic field, thereby increasing the carrier generation due to impact ionization. This leads to a lower blocking voltage. Moreover, the conventional devices use oxides deposited on the surface of the devices, wherein these surfaces are very close to this high electrostatic field region, leading to additional breakdown. Furthermore, because the mesa region is narrower than the substrate, the current density is very high near the location of high electrostatic field, leading to enhanced impact ionization.
Therefore, there remains a need for further development of silicon carbide GTO thyristors capable of blocking increased high voltages, which also overcome the deficiencies of the conventional devices.
In view of the foregoing, an embodiment of the invention provides a device that is capable of blocking increased voltages by using a multi-layered thick drift region. Moreover, an embodiment of the invention provides a silicon carbide GTO thyristor having improved voltage blocking characteristics. Specifically, an embodiment of the invention provides a silicon carbide GTO thyristor in which the drift region contains multiple layers of different dopant concentrations to reshape the electrostatic field, wherein the highest magnitude of the electrostatic field is located in a predetermined location within the thyristor which produces better voltage blocking results. Additionally, an embodiment of the invention provides a silicon carbide GTO thyristor in which the maximum electrostatic field in the thyristor occurs at a much higher applied voltage.
An embodiment of the invention provides a structure for a voltage blocking device comprising a cathode region, a drift region positioned on the cathode region, a gate region positioned on the drift region, an anode region positioned on the gate region and a plurality of contacts positioned on each of the cathode region, the gate region, and the anode region, wherein the drift region comprises multiple epilayers having first doped type layers surrounding second doped type layers, wherein dopant concentrations of the first doped type layers are lower than dopant concentrations of the second doped type layers. The epilayers comprise at least one i-n-i layer and/or at least one i-p-i layer. Moreover, the device comprises silicon carbide. Furthermore, the device is a gate turn-off (GTO) thyristor, or the device comprises any one of an insular gate bipolar transistor, a silicon controlled rectifier, a pin diode, a gate turn-off device, and a metal oxide semiconductor turn-off device. Additionally, the multiple epilayers are operable to block voltages in the device.
Alternatively, an embodiment of the invention provides a structure for a silicon carbide thyristor device comprising a cathode region, a drift region positioned on the cathode region, a gate region positioned on the drift region, an anode region positioned on the gate region, and a plurality of ohmic contacts positioned on each of the cathode region, the gate region, and the anode region, wherein the drift region comprises multiple doped layers comprising n-type and p-type dopants and having first doped type layers surrounding second doped type layers, wherein dopant concentrations of the first doped type layers are lower than dopant concentrations of the second doped type layers, and wherein the multiple doped layers are operable to block voltages in the device. The multiple doped layers comprise at least one i-n-i layer and/or at least one i-p-i layer. Furthermore, the device is a gate turn-off (GTO) thyristor, or any one of an insular gate bipolar transistor, a silicon controlled rectifier, a pin diode, a gate turn-off device, and a metal oxide semiconductor turn-off device.
Additionally, an embodiment of the invention provides a method of increasing a voltage threshold in a voltage blocking device comprising a cathode region adjacent a drift region, the drift region adjacent a gate region, and the gate region adjacent an anode region, wherein the method comprises adding additional epilayers to the drift region, wherein each one of the epilayers comprises a predetermined dopant concentration. The epilayers comprise at least one i-n-i layer and/or at least one i-p-i layer. The step of adding additional epilayers to the drift region causes an electrostatic field located in the device to move away from an upper portion of the drift region. Moreover, the multiple epilayers block voltages in the device.
The embodiments of the invention achieve several advantages. For example, the invention reduces the number of processing steps involved in forming a high voltage blocking device because the lithography and deposition steps needed for field rings and field plates, or the lithography, implant and annealing processing steps required of guard rings or junction termination extensions can be avoided. An embodiment of the invention also makes it possible to develop high voltage blocking devices by using a thinner overall drift region which has the added benefit of lower cost and better material quality. In fact, because the material quality worsens as an epilayer becomes thicker, which is a problem inherent in the conventional devices, an embodiment of the invention overcomes this deficiency by providing thinner multiple layers.